This is a 371 of PCT/JP94/01202 filed Jul. 21, 1994 and published as WO95/03351 Feb. 2, 1995.
1. Field of The Invention
The present invention relates to a method for producing an aromatic polycarbonate.
2. Prior Art
In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound, such as 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as "bisphenol A"), and phosgene has been commercially practiced.
However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous, that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride, which are by-produced, and methylene chloride which is used as a solvent in a large quantity, and that difficulties are encountered in separating and removing impurities, such as sodium chloride, and residual methylene chloride, which adversely affect properties of a produced polymer.
With respect to a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, in a conventionally known melt polycondensation process a polycarbonate is produced by performing an ester exchange reaction between bisphenol A and diphenyl carbonate in the molten state, while removing a by-produced phenolic compound (phenol). Unlike the interfacial polycondensation process, the melt polycondensation process has an advantage in that a solvent need not be used. However, the melt polycondensation process has a serious problem, namely; since the viscosity of polymer being formed increases during the progress of the polymerization reaction, it becomes difficult to remove by-produced phenol from the polymerization reaction system efficiently, thus making-it difficult to achieve a high degree of polymerization with respect to polycarbonate produced.
Various polymerizers have been known for use in producing aromatic polycarbonates. An agitation type polymerizer vessel equipped with an agitator is widely used. The agitation type polymerizer vessel equipped with an agitator is advantageous in that it exhibits high volumetric efficiency and has a simple construction, so that polymerization on a small scale can be efficiently carried out. However, the agitation type polymerizer vessel has a problem in that, as mentioned above, the by-produced phenol becomes difficult to remove from the polymerization reaction system efficiently in the production of aromatic polycarbonates on a commercial scale, so that it is difficult to achieve a high degree of polymerization with respect to produced polycarbonate.
Specifically, a large-scale agitation type polymerizer vessel generally has a greater ratio of liquid volume to vaporization area than a small-scale one. In other words, the depth of a reaction mixture in the polymerizer is large. In such a case, even if the degree of vacuum of the polymerization reaction zone is raised in order to achieve a high degree of polymerization in the lower part of the agitation vessel, the polymerization proceeds under virtually high pressure due to the weight of the reaction mixture, so that phenol and the like cannot be efficiently removed.
To solve the above-mentioned problem, various attempts have been made to remove phenol and the like from high viscosity polymer being formed. For example, Examined Japanese Patent Application Publication No. 50-19600 discloses a screw type polymerizer having a vent. Examined Japanese Patent Application Publication No. 53-5718 discloses a thin film evaporation type reactor, such as a screw evaporator and a centrifugal film evaporator. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923 discloses a method in which a combination of a thin film evaporation type apparatus and a horizontal stirring polymerization vessel is used. These polymerizers, including those of an agitation type polymerizer vessel, have a common drawback in that they have a rotary driving part in the main body, which rotary driving part cannot be completely sealed, so that when the polymerization is conducted under high vacuum, a small amount of oxygen inevitably leaks into the reaction system, leading to discoloration of final polymer. When a sealant is used to prevent the leak-in of oxygen into the reaction system, the sealant unavoidably gets mixed into the final polymer, so that the quality of the final polymer is lowered. These polymerizers also have a serious maintenance problem in that, even if the seal effect is high at the beginning of the operation of the polymerizer, the seal effect is inevitably lowered during the continuous operation for a prolonged period of time.
A free-fall polymerization process, in which polymerizing material is allowed to pass downwardly through a perforated plate and fall freely, so that polymerization of the polymerizing material is effected during the free-fall (in this process there is no need of using a polymerizer having a rotary driving part in a main body thereof), is known as a method for producing resins other than aromatic polycarbonates. For example, U.S. Pat. No. 3,110,547 discloses a method for producing a polyester having desired molecular weight, in which polyester having a low degree of polymerization is allowed to fall freely in the form of filaments through a vacuum zone. In the technique of this U.S. Patent, since recirculation of the fallen polymer and repetition of the free fall causes a lowering of the quality of the final polyester, the polymerization is finished upon one-time free-fall without recirculation. However, with respect to such a method, many drawbacks have been pointed out. For example, concerning a method of spinning a polyester having a low degree of polymerization through a spinneret into a vacuum zone to effect polycondensation thereof, Examined Japanese Patent Application Publication No. 48-8355 contains a description such that when polymerizing material (not having a satisfactorily high spinnability) is fed into a reactor, filaments being polymerized are likely to be broken, so that the quality of a polycondensate is drastically lowered. Low molecular weight polycondensate scattering from the filaments sticks to the surface of the spinneret to smudge the spinneret and, hence, it becomes difficult for the filaments to fall straight down through the spinneret, so that the filaments are caused to contact each other to bring about breakage of the filaments or are caused to join one another, thus hindering the polymerization reaction. Further, observation windows easily get clouded and, hence, observation becomes difficult, so that an observer has difficulty in ascertaining an appropriate time for replacement of smudged spinnerets with fresh ones. In the above Japanese patent document, it is further described that, for the above reasons, when producing a polyester and a polyamide, it is preferred to employ a non-free-fall process in which a polymer having a low degree of polymerization is allowed to flow down along and in contact with a porous material arranged vertically in a reaction vessel. However, the above Japanese patent document contains no description about aromatic polycarbonates.
Aside from a polymerization method, with respect to a method for removing a residual monomer from polymerization products, U.S. Pat. No. 2,719,776 proposes a process of spinning a lactam polymerization product, which comprises allowing the product to pass through a perforated plate and fall freely in the form of filaments, whereby the residual monomer is removed by evaporation. However, many disadvantages accompanying this method have been pointed out. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 53-17569 points out that the method of U.S. Pat. No. 2,719,776 has various disadvantages. That is, in the method of the above-mentioned U.S. Patent, when the evaporation of the volatiles is small, filaments can be formed, whereas when the evaporation of the volatiles is large, filaments unfavorably suffer foaming, making it difficult to carry out the monomer-removing operation smoothly. Further, this method can be applied only to a polymerization product having a viscosity in a relatively narrow range suitable for forming filaments. Moreover, in this method, when an inert gas is introduced into a column in which this method is practiced, filaments are caused to contact and join one another due to the turbulence of the flow of inert gas. To solve such disadvantages, Unexamined Japanese Patent Application Laid-Open Specification No. 53-17569 proposes a non-free-fall process which comprises providing a linear support arranged vertically, and allowing a high viscosity material to fall along and in contact with the linear support. This Japanese patent document proposes this non-free-fall process as a method for producing polyesters, such as polyethylene terephthalate and polybutylene terephthalate, and polyamides, such as nylon 6 and nylon 66. However, in this Japanese patent document, there is no mention of aromatic polycarbonates.
Examined Japanese Patent Application Publication No. 4-14127 has a description to the effect that, as a continuous polycondensation process for producing a polyester, there have been known two types of free-fall polycondensation processes, namely: a spinning process in which a polymerizing material is spun through a spinneret and a process in which a polymerizing material is extruded in the form of a film through a slit to effect polymerization thereof, and that, however, both processes have difficulties in obtaining a polymer having a satisfactorily high molecular weight. This Japanese patent document proposes a non-free-fall method in which a polymerizing material is allowed to pass through a slit-shaped feed port, and the resultant film is supported between at least two wires which are vertically and separately arranged and allowed to move downwardly between the wires, thereby effecting polycondensation of the polymerizing material. In this Japanese patent document, no mention is made of aromatic polycarbonates.
As described above, a free-fall polymerization process, in which polymerizing material is allowed to pass through a perforated plate and fall freely, is known as a method for producing polyesters and polyamides, but such a process is not known at all as a method for producing aromatic polycarbonates. Further, many drawbacks have been pointed out with respect to the free-fall polymerization processes for producing polyesters and polyamides. Therefore, various types of non-free-fall polymerization processes have been proposed for producing polyesters and polyamides.
It is a primary object of the present invention to provide a melt polycondensation process for producing a colorless and high quality aromatic polycarbonate at a high polymerization rate and stably for a prolonged period of time, using apparatus which is excellently sealed under high vacuum conditions, and easy to maintain.